The invention relates to nuclear medicine, and more particularly relates to images produced by nuclear medicine studies. In its most immediate sense, the invention relates to nuclear medicine studies of organs, particularly studies of the heart.
Other modalities, such as computed tomography and magnetic resonance imaging, produce clearly defined images. However, in nuclear medicine studies, e.g. cardiac bloodpool studies (which are more concerned with functional determinations such as ejection fractions rather than location of malignant regions as in other studies) it is necessary to identify such organ features as the interventricular septum, the left ventricle and the right ventricle. Since cardiac tissue is engorged with blood and the tissue adjoins blood pools within the heart, there are no sharp edges (as exist with images produced by other modalities) and even under the best of circumstances it is difficult to locate exactly that portion of the image which relates to, e.g., the left ventricle.
Attempts have been made to use edge detection methods to identify where anatomic features are located, but such attempts have been unsuccessful. This is because the data used to construct such images is contaminated by scatter and attenuation. Additionally, nuclear medicine images become even more ambiguous because of low resolution, low signal-to-noise ratio and the presence of radioactivity from adjacent organs and background tissue. Consequently, conventional detection methods are not helpful in identifying anatomical structures of interest; these methods may locate an edge which is without diagnostic significance and may overlook an edge which is highly important.
Additionally, it frequently happens that the camera is mispositioned with respect to the patient's target organ (i.e. the organ of interest). (This may happen where the patient has, e.g., an abnormally oriented heart, or where the patient shifts position after being positioned properly.) Even an experienced technician needs substantial time to collect enough data to assess the positioning of the patient and to correct any mispositioning, and an inexperienced technician may need to repeat these steps one or more times before the patient is positioned properly. Worse, if the mispositioning is not detected and a study has been conducted with the patient in a suboptimal position, the diagnostician is forced to choose between using the resulting suboptimal study (which is of diminished diagnostic utility) or repeating it (and thereby dosing the patient once again and decreasing throughput through the camera).
It would therefore be advantageous to provide method and apparatus which would be capable of automatically identifying anatomic features in nuclear medicine images for the target organ in nuclear medicine studies even where the target organ is not sharply defined.
It would also be advantageous to provide method and apparatus which could automatically identify the camera positioning between the patient's target organ and the camera detector far in advance of the end of the study, thereby permitting manual or even automatic repositioning of the camera head to an optimal position.
One object of the invention is to provide method and apparatus which can identify anatomic features in nuclear medicine images of target organs.
Another object of the invention is to provide method and apparatus which can automatically identify anatomic landmarks in nuclear medicine images, even when the images contain insufficient data to be diagnostically useful.
A further object of the invention is to provide method and apparatus which will permit automatic positioning of the scintillation camera detector.
Still another object is, in general, to improve on known methods and apparatus used in nuclear medicine.
The invention proceeds from the realization that conventional edge detection methods are unsuitable for use in nuclear medicine images. This is because such methods do not incorporate any anatomy-specific constraints and therefore cannot distinguish between patterns having diagnostic significance and patterns which lack diagnostic significance. Additionally, because nuclear medicine images of tissue are not sharply focussed even under the best of circumstances, it is difficult for edge detection methods to produce meaningful edge curves.
However, it is possible to take advantage of a basic characteristic of nuclear medicine in order to locate anatomical landmarks. In a nuclear medicine study, the uptake of a radioactive isotope determines the intensity of the nuclear medicine image and there are no shadows, reflections, or artificial highlights. Therefore, segments of maximum and minimum intensity will in certain instances be intrinsic to the patient's anatomy and will be unaffected by changes in imaging conditions. As a result, it is possible to utilize the relative locations of such maxima and minima in order to locate anatomical landmarks.
In accordance with the invention, a nuclear medicine image is scanned line by line. For the intensity profile of each scan line, local curvatures are computed to identify local intensity maxima and local intensity minima. After the whole image has been so processed, line segments are constructed from all identified local intensity maxima, and other line segments are constructed from all identified local intensity minima. Then, these line segments are evaluated to see whether they satisfy constraints which are specific to the anatomical region of interest. If so, the line segments are treated as identifying particular anatomical landmarks, and on the basis of these landmarks, boundaries for the target organ or the portion of interest of the target organ can be easily located.
For example, if a cardiac image shows the left and right ventricles separated by the interventricular septum, there will be line segments constructed from pixels of maximum intensity near the long axis of each ventricle and there will be a line segment constructed from pixels of minimum intensity running near the interventricular septum and between the two line segments of maximum intensity. Furthermore, this "maximum-minimum-maximum" pattern will have a certain spatial relationship which is dictated by the structure of the heart. Therefore, "maximum-minimum-maximum" patterns which are heart-related can be distinguished from similar patterns which are not heart-related. Consequently, a computer can be programmed to scan a cardiac gated blood pool image and to draw the above-mentioned line segments (anatomical landmarks).
It is possible, in accordance with the invention, to more easily deduce the boundaries of the ventricles of the heart from the above-mentioned line segments because there are certain geometrical relationships which are intrinsic to the heart and the boundaries can be expected to lie within regions which can be defined by these anatomical landmarks. It is therefore possible to limit the identifications of anatomical features of interest to comparatively small regions which exclude anatomical structures that are not of interest.
Furthermore, because the referenced "maximum-minimum-maximum" pattern is dictated by the structure of the heart, that pattern exists at all stages of image acquisition even if there is insufficient data to form an image which would be diagnostically useful. There is a one-to-one relationship between the detected landmark pattern and the camera position. Thus, even after a relatively short time, the position of the camera detector with respect to the patient's target organ can be inferred from the relative positioning of the above-mentioned line segments. From this, it can be determined whether the camera has been properly positioned and how the camera should be repositioned; this permits the camera to be repositioned so that a study is not conducted at a suboptimal angle.
Additionally, a camera with a motor-driven gantry can be adapted to automatically position itself to an optimal position. With such a gantry, the camera detector can be stepped around the patient at small angular increments (each step position is known as a camera stop), with one frame of information being acquired at each position. In further accordance with the invention, after relevant frames of information have been scanned, the inferred camera position is registered.